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An opus number (Latin sing. "work", "labour", a work of art, 1695–1705), pl. opera and opuses, abbreviated, sing. Op. and pl. Opp. refers to a number generally assigned by composers to an individual composition or set of compositions on publication, to help identify their works. Opus numbers have been used inconsistently throughout history and by individual composers, and thus are not generally a good indicator of the chronological order of the compositions or the relative completeness of a given collection. Opus numbers are commonly used to organise catalogues of musical compositions. The Latin plural, opera, also denotes the opera music genre, wherein the works also are (occasionally) identified with a musical composition opus number, as in the grand opera Samson and Delilah, Op. 47, by Camille Saint-Saëns.
Early usage 
In the arts, opus number usually denotes a work of musical composition, a practice and usage established in the seventeenth century when composers identified their works with an opus number. In the eighteenth century, publishers usually assigned opus numbers when publishing groups of like compositions, usually in sets of three, six or twelve compositions. Consequently, opus numbers are not usually in chronological order, unpublished compositions usually had no opus number, and numeration gaps and sequential duplications occurred when publishers issued contemporaneous editions of a composer’s works, as in the sets of string quartets by Joseph Haydn (1732–1809) and Ludwig van Beethoven (1770–1827); Haydn's Op. 76, the Erdödy quartets (1796–97), comprises six discrete quartets consecutively numbered Op. 76 No. 1 – Op. 76 No. 6; whilst Beethoven's Op. 59, the Rasumovsky quartets (1805–06), comprises String Quartet No. 7, String Quartet No. 8, and String Quartet No. 9.
19th century to date 
From about 1800, composers, especially Beethoven, assigned an opus number to a work, and later to a set of works, especially songs and short piano pieces; however, composers’ inconsistent usages ended the correspondence between an opus number and the work’s publication date. Since approximately 1900, composers tended to assign an opus number to a composition, published or not. Early in his career, Beethoven selectively numbered his compositions (some published without opus numbers), yet in later years, he published early works with high opus numbers. Likewise, some posthumously published works were given high opus numbers by publishers, even though some of them were written early in Beethoven's career. Since his death in 1827, the un-numbered compositions have been catalogued and labelled with the German acronym WoO (Werk ohne Opuszahl), meaning "work without opus number". However, there are other catalogues of Beethoven's works - see Catalogues of Beethoven compositions.
The practice of enumerating a posthumous opus (“Op. posth.”) is noteworthy in the case of Felix Mendelssohn (1809–47); after his death, the heirs published many compositions with opus numbers that Mendelssohn did not assign them. In life, he published three symphonies (Symphony No. 1 in C minor, Op. 11; Symphony No. 2 in B flat major, Op. 52; and Symphony No. 3 in A minor, Op. 56); yet, he chronologically wrote symphonies between symphonies Nos. 1 and 2, which he withdrew for personal and compositional reasons; nevertheless, the Mendelssohn heirs published (and catalogued) them as the Italian Symphony No. 4 in A major, Op. 90, and as the Reformation Symphony No. 5 in D major and D minor, Op. 107.
While many of the works of Antonín Dvořák (1841–1904) were given opus numbers, these did not always bear a logical relationship to the order in which the works were written or published. To achieve better sales, some publishers, such as N. Simrock, preferred to present budding composers as being well established, by giving some relatively early works much higher opus numbers than their chronological order would merit. In other cases, Dvořák gave lower opus numbers to new works to be able to sell them to other publishers outside his contract obligations. This way it could happen that the same opus number was given to more than one of his works. Opus number 12, for example, was assigned, successively, to five different works (an opera, a concert overture, a string quartet, and two unrelated piano works). In other cases, the same work was given as many as three different opus numbers by different publishers. The sequential numbering of his symphonies has also been confused: (a) they were initially numbered by order of publication, not composition; (b) the first four symphonies to be composed were published after the last five; and (c) the last five symphonies were not published in order of composition. The New World Symphony originally was published as No. 5, later was known as No. 8, and definitively was renumbered as No. 9 in the critical editions published in the 1950s.
Other examples of composers' historically inconsistent opus-number usages include the cases of César Franck (1822–1890) and Béla Bartók (1881–1945), who initially numbered, but then stopped numbering, their compositions. Carl Nielsen (1865–1931) and Paul Hindemith (1895–1963) were also inconsistent in their approaches. Sergei Prokofiev (1891–1953) was consistent and assigned an opus number to a composition before composing it; at his death, he left fragmentary and planned, but numbered, works. In revising a composition, Prokofiev occasionally assigned a new opus number to the revision; thus Symphony No. 4 is two thematically related but discrete works: Symphony No. 4, Op. 47, written in 1929; and Symphony No. 4, Op. 112, a large-scale revision written in 1947. Likewise, depending upon the edition, the original version of Piano Sonata No. 5 in C major, is catalogued both as Op. 38 and as Op. 135.
Alternative catalogues 
To deal with inconsistent opus number usage, especially by those of the baroque and classical eras, musicologists have comprehensively catalogued the works of many composers and allocated unambiguous new catalogue designations to them. Among the best known of these are:
- Johann Sebastian Bach's works are referred to by BWV (Bach-Werke-Verzeichnis) number assigned in the catalogue by Wolfgang Schmieder. Older sources sometimes use S numbers.
- Dietrich Buxtehude's works have received BuxWV numbers, for Buxtehude-Werke-Verzeichnis.
- Frédéric Chopin: Three major attempts have been made to catalogue all Chopin's works systematically. Maurice J. E. Brown uses B numbers, Krystyna Kobylańska uses KK numbers, and Józef Michał Chomiński allocates numbers to a variety of letters (A, C, D, E, P and S). These alternative designations are generally used to identify only the works that were not given opus numbers.
- Claude Debussy's works are usually referred to by the L or Lesure numbers, after François Lesure's comprehensive catalogue.
- Antonín Dvořák's works are usually now referenced by B numbers, after Jarmil Burghauser's comprehensive catalogue which resolved a great many difficulties with the often misleading and duplicated opus numbers given by different publishers to Dvořák's works.
- Joseph Haydn's works are referred to by their Hob. or Hoboken numbers after Anthony van Hoboken's 1957 classification. Hoboken assigned numbers to the string quartets, but these are generally still known by their opus numbers.
- Franz Liszt's works are referred to by their S or Searle numbers after Humphrey Searle's 1960s classification The Music of Liszt.
- Wolfgang Amadeus Mozart's works are referred to by their K or Köchel numbers, after Ludwig Ritter von Köchel. In continental Europe, the German abbreviation KV for Köchel-Verzeichnis is more common.
- Domenico Scarlatti's works can appear under three different catalogue systems. Alessandro Longo's 1906 catalogue (L numbers) was superseded by Ralph Kirkpatrick's 1953 catalogue (K or Kk numbers). Giorgio Pestelli's 1967 catalogue (P numbers) also has its adherents.
- Franz Schubert's works are referred to by their D or Deutsch numbers after Otto Erich Deutsch's catalogue.
- opus, n. (June 2008). The Oxford English Dictionary Online. Oxford University Press. Retrieved 31 August 2009 <http://dictionary.oed.com/cgi/entry/00333025>.
- opus. The American Heritage Dictionary of the English Language, 4th edition.